Excessive B cell activity and loss of regulatory cells occurs in patients with active SLE; these abnormalities are less marked when the disease is quiescent. Nevertheless, excessive B cell proliferation occurs even in inactive patients. Loss of a subpopulation of regulatory T cells occurs in patients with active SLE. Antibodies to the precursors of suppressor T cells are spontaneously produced by the patients. Such antibodies prevent normal T cell control of abnormal B cell activity allowing perpetuation of autoantibody production. Some patients produce antibodies to a subset of T cells which regulates T cells but not B cell immunity. Such patients are less ill than those which produce antibodies capable of interfering with regulation of B cell function. The same patients which have impaired suppressor T cell function when active regain normal suppressor T cell function when the disease is quiescent. Studies with monoclonal anti-T cell antibodies indicate that during active disease there is a quantitative decrease in suppressor cells which is not present when the disease is inactive.